The effects were examined of 6-month intermittent hypobaric (4000 m) exposure on the antioxidant enzyme systems in soleus and tibialis muscles of rats. At the end of the 6-month experimental exposure, the six rats in both the exposed group and the control group were sacrificed. Immunoreactive mitochondrial superoxide dismutase (Mn-SOD) contents were measured as well as the activities of antioxidant enzymes [Mn-SOD, cytosolic SOD (Cu,Zn-SOD), catalase (CAT), and glutathione peroxidase (GPX)]. Thiobarbituric acid-reactive substances (TBARS) were also determined as an indicator of lipid peroxidation. The high altitude exposure resulted in a marked increase in TBARS content in soleus muscle, suggesting increased levels of oxygen free radicals. Conversely, significant decreases in both Mn-SOD content and activity in soleus muscle were noted after exposure. Such trends were not noticed in tibialis muscle. On the other hand, no significant changes in Cu,Zn-SOD, CAT, or GPX were observed in either muscle. These results suggested that the increases in lipid peroxidation were most probably a result of decreased Mn-SOD function which was more depressed in oxidative than in glycolytic muscle.
The measurements on strong quasiparticle injection through a tunnel barrier into a high-T, YBa2Cu307 (YBCO) bicrystal film/junction or a YBCO film are presented. With an increase of tunnel injection current, both the Josephson and film critical currents decreased and were driven to zero to yield the normal state at a certain critical injection current density which ranged 20-140 A/cm for 70 to a few 100 mV voltage bias. The results are qualitatively different from simple heating phenomenon and reflect nonthermal nature of high-T, superconductors under injection.The quasiparticle injection phenomenon into a nonequilibrium superconductor received considerable attention for low-T, superconducting materials. ' The quasiparticle injection is held either by optical excitation or tunnel injection of quasiparticles or phonon injection. Among them, the intense tunnel injection yielded a variety of interesting phenomenon such as the gap suppression, the instability toward the spatial inhomogeneous states9 and the multiple gap structures, ' and the possible applications to electronics. ' ' The 'quasiparticle injection into a high-T, superconductor has been so far held by optical excitation technique. ' ' It has been reported that both bolometric and nonbolometric (nonequilibrium) responses were obtained for a high-T, superconductor, however, their essential natures are still controversial. The observed quasiparticle recombination time was 2-S psec. ' In the case of a high-T, superconductor, because of its large gap parameter and high T" it is diScult to drive it to a strongly perturbed nonequilibrium state. In fact, there are very few reports on this matter.In this paper, we report the measurements of tunnel injection of quasiparticles into a YBa2Cu307 (YBCO) film and a YBCO bicrystal film/junction.In contrast to optical excitation (excitation energy 1 eV), tunnel injection corresponds to injection of quasiparticles with energy 20 meV to a few 100 meV. The quasiparticle injection through a tunnel barrier led to clear reduction of bicrystal Josephson current and YBCO film critical current. At a certain injection current, the supercurrent completely disappeared to yield the normal state at 4.2 K. The critical injection current density ranged 20 -140 A/cm2. It was found that the observed phenomena were qualitatively difFerent from simple heating phenomenon and were considered to be of essentially nonequilibrium nature.We have used two types of sample geometries as shown in Fig. 1. One is an Au/MgO/YBCO junction grown on a SrTi03 (100) bicrystal substrate (Dowa Mining Co. Ltd. ) (a) and the other is that grown on a MgO (100) substrate. The crystal angle at the interface of the bicrystal substrate was 24 . Sample (a) contains a bicrystal Josephson junction. The samples were fabricated by in situ sequential deposition of trilayer films utilizing an C = & Ilnj AU MBO / Yeco rrrrrrrrrrrrrrrrr,
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